1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer module and a production method therefor.
2. Description of the Related Art
The widespread use of a communication network using optical fiber transmission is essential for the construction of multimedia information society, and it is therefore necessary to realize a photodetector module for efficiently converting an optical signal into an electrical signal at a low cost. Further, wavelength division multiplex optical communication for multiplexing a plurality of optical signals having different wavelengths to transmit wavelength division multiplexed signal light by taking advantage of the wavelength dependence of light is being expected to provide various services including bidirectional communication and service multiplex, and it is therefore essential to provide an optical multiplexer/demultiplexer module for multiplexing and demultiplexing optical signals having different wavelengths.
A coaxial type optical multiplexer/demultiplexer module and a box type optical multiplexer/demultiplexer module are known in the art. The coaxial type optical multiplexer/demultiplexer module includes a cylindrical body, various optical components such as a bandpass filter and an optical isolator fixed in the cylindrical body by bonding or soldering, and input/output optical fibers welded to the opposite ends of the cylindrical body by YAG laser. The box type multiplexer/demultiplexer module includes a box, various optical components such as a bandpass filter and an optical isolator bonded or welded to the bottom surface of the box, and input/output fiber lens assemblies are welded to the side surfaces of the box after alignment. In each type optical multiplexer/demultiplexer module, the fiber and the lens in each of input and output ports must be adjusted in advance so that a collimated beam is generated. The various optical components are arranged in the optical path of the collimated beam, and fixed in the cylindrical body or at the bottom of the box by bonding or soldering.
Referring to FIG. 1, the structure of a coaxial type optical multiplexer/demultiplexer module 2 in the prior art will further be described. Reference numeral 4 denotes a cylindrical body. An input port 6 is fixed to one end of the cylindrical body 4, and an output port 8 is fixed to the other end of the cylindrical body 4. The input port 6 includes a sleeve 16, a lens holder 12 inserted and fixed in the sleeve 16, a spherical lens 14 press-fitted in the lens holder 12, and a ferrule 10 inserted and fixed in the lens holder 12. The sleeve 16 is spot-welded to one end of the cylindrical body 4.
The output port 8 includes a sleeve 24, a lens holder 22 inserted and fixed in the sleeve 24, a spherical lens 20 press-fitted in the lens holder 22; and a ferrule 18 inserted and fixed in the lens holder 22. A semispherical block 26 is fixed to the other end of the cylindrical body 4, and the output port 8 is welded to the semispherical block 26 after alignment. Various optical components 28, 30, 32, and 34 such as a bandpass filter and an optical isolator are accommodated in the cylindrical body 4.
In view of the properties of the optical components used in the optical multiplexer/demultiplexer module, it is essential to ensure optical coupling by a collimated beam and suppress optical path deviation. In the coaxial type optical multiplexer/demultiplexer module 2 as shown in FIG. 1, however, the deviation of an optical path occurs because of a difference in refractive index and thickness among the various optical components 28, 30, 32, and 34. Reference numeral 36 denotes the optical path thus deviated.
The coaxial type optical multiplexer/demultiplexer module in the prior art has the following problems.
(1) It is difficult to correct the optical path deviation because of a limited narrow space.
(2) It is difficult to connect the multiple optical components in tandem because of the occurrence of the optical path deviation. In particular, the angle of incidence on the module is large in the case of using a two-core ferrule, so that the output port must be largely adjusted in angle, resulting in that the output port largely projects from the outer diameter of the cylindrical body.
(3) The workability is low because the optical components are mounted into the cylindrical body.
(4) In the case of using a method of fixing the optical components inserted in the cylindrical body by through welding, the tolerance becomes close to cause an increase in parts cost.
On the other hand, the box type optical multiplexer/demultiplexer module in the prior art has the following problems.
(1) It is difficult to reduce the size.
(2) The box is a member formed by cutting, so that the parts cost is high.
(3) The workability in fixing the optical components and in alignment is low.
Further, in the case of adjusting the angle of emergence of output light from the output port, the output light is monitored by an infrared camera or the like, and the infrared camera is moved along the optical axis of the module to calculate the emergent angle of the output light from the amount of movement of the camera and the deviation of an image obtained by the camera. However, this method can make accurate measurement only in the case of a collimated beam. In the conventional method, it is necessary to measure convergent light passed through the lens, so that accurate measurement and adjustment are impossible. Further, in adjusting the lens, the measurement is required at two or more points as moving the camera, causing an increase in adjustment time.
It is therefore an object of the present invention to provide an optical multiplexer/demultiplexer module which can correct the deviation of an optical path occurring in combining a plurality of elements in tandem.
It is another object of the present invention to provide a production method for such an optical multiplexer/demultiplexer module improved in productivity.
In accordance with an aspect of the present invention, there is provided an optical multiplexer/demultiplexer module comprising a first ferrule assembly having a first sleeve and a first ferrule inserted and fixed in the first sleeve, the first ferrule having first and second optical fibers; a first element fixed to the first ferrule assembly, the first element having a first lens and a first bandpass filter; a second element fixed to the first element, the second element having a wedge plate for optical path correction, the wedge plate being formed of a material transparent to the wavelength of light to be used; a third element fixed to the second element, the third element having a second lens; and a second ferrule assembly fixed to the third element, the second ferrule assembly having a second sleeve and a second ferrule inserted and fixed in the second sleeve, the second ferrule having a third optical fiber.
Preferably, the optical multiplexer/demultiplexer module further includes a fourth element interposed between the first element and the third element, the fourth element having a second bandpass filter. Preferably, the wedge plate has a first flat surface and a second flat surface opposite to the first flat surface, the second flat surface being inclined with respect to the normal to the optical axis of the module by a given angle. The direction of inclination of the second flat surface of the wedge plate coincides with the direction of extension of a plane defined by the first and second optical fibers of the first ferrule. Preferably, the first lens includes an aspherical lens, and the wedge plate is formed of glass. Each of the first and second bandpass filters includes a glass plate and a dielectric multilayer film formed on the glass plate.
In accordance with another aspect of the present invention, there is provided a production method for an optical multiplexer/demultiplexer module, comprising the steps of preparing a first element having a first lens, a first bandpass filter, and a marker; preparing a second element having a wedge plate for optical path correction and a marker, the wedge plate being formed of a material transparent to the wavelength of light to be used and having a flat surface inclined with respect to the normal to the optical axis of the module by a given angle; fixing the second element to the first element after alignment of the markers of the first and second elements; preparing a first ferrule assembly having a first sleeve and a first ferrule inserted and fixed in, the first-sleeve, the first ferrule having first and second optical fibers; fixing the first ferrule assembly to the first element after aligning the first ferrule assembly so that the direction of inclination of the flat surface of the wedge plate coincides with the direction of extension of a plane defined by the first and second optical fibers; preparing a third element having a second lens; fixing the third element to the second element after aligning the third element to the second element; preparing a second ferrule assembly having a second sleeve and a second ferrule inserted and fixed in the second sleeve, the second ferrule having a third optical fiber; and fixing the second ferrule assembly to the third element.
Preferably, the production method further includes the steps of preparing a fourth element having a second bandpass filter and a fourth marker; and fixing the fourth element to the second element after alignment of the second and fourth markers.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.